The present invention relates to voltage-controlled oscillators and in particular to a CMOS voltage-controlled oscillator which operates over a wide range of frequencies.
As computers and computer implemented applications become more sophisticated, there is a growing demand for systems which can process a high volume of data very quickly. High volume data processing may be implemented by increasing the amount of data that a given system processes, by increasing the speed at which a given system processes a set amount of data, or by combination of these two methods.
Examples of processes which benefit from rapid high volume data processing are those which display high quality graphics and those which operate on relatively large data sets.
In addition to handling these high data rate processes, it is desirable for computer systems to continue to handle existing lower data rate processes. One example of this need for wide-range processing is image display hardware and software for IBM compatible personal computers. Existing display adapters for these computers support graphics formats ranging from 64,000 pixels per image to 1,310,720 pixels per image, and image refresh rates from 60 Hz to 87 Hz. To support all of these formats, one of these systems may require pixel clock signals having frequencies which range from 4 MHz to 120 MHz. With information displays increasing in size there will be demand for even higher pixel display frequencies.
To accommodate these display rates, a flexible pixel clock signal generator is desirable. Ideally, this clock signal generator should change the frequency of the pixel clock signal quickly and with few visible artifacts.
Another area in which high data rate processing is in demand is in accessing data from memory. To meet this demand, new memory architectures are also being developed. These architectures allow high volumes of data to be accessed very quickly. One exemplary architecture is the RAMBUS system available from Toshiba Corporation and Nippon Electric Corporation. This system can provide data transfer rates of up to 5 MB per second. To achieve this high data transfer rate, this system employs a 250 MHz clock signal and transfers data on both the positive going and negative going edges of a clock.
Existing systems which provide clock signals for computer systems employ oscillators which may be switched to operate in different bands of frequencies. One such system is disclosed in U.S. Pat. No. 5,036,216 entitled "VIDEO DOT CLOCK GENERATOR". This system uses a conventional ring oscillator fabricated using MOS technology but switches capacitances into the ring oscillator to change the speed at which signals propagate from one stage of the oscillator to the next. A decrease or increase in the capacitances causes a corresponding increase or decrease the frequency of the signal produced by the oscillator. This idea was extended in U.S. Pat. No. 5,302,920 entitled "CONTROLLABLE MULTI-PHASE RING OSCILLATORS WITH VARIABLE CURRENT SOURCES AND CAPACITANCES". The system described in this patent allows a plurality of capacitances to be switched in the ring oscillator, thereby allowing multiple frequency ranges.
While these programmable ring oscillators work well in existing systems, they may not be able to realize the broad range of frequency which may be required for future applications without adding undesirable distortion to the signals that are synchronized to these oscillators.